U.S. patent application number 09/774398 was filed with the patent office on 2002-08-01 for porous polymeric substrate treatment device and method.
Invention is credited to Bahten, Kristan G., Catalano, Vincent J., Innes, James S., McMullen, Daniel T..
Application Number | 20020100132 09/774398 |
Document ID | / |
Family ID | 25101114 |
Filed Date | 2002-08-01 |
United States Patent
Application |
20020100132 |
Kind Code |
A1 |
McMullen, Daniel T. ; et
al. |
August 1, 2002 |
Porous polymeric substrate treatment device and method
Abstract
A device for treating surfaces of substrates utilized in
electronics manufacturing includes a resilient skin encasing a
porous polymeric interior. The resilient skin overlies the exterior
surface of the brush, and is typically characterized by a higher
density, a smaller pore size, and a lower porosity than the
interior material of the brush. The skin may serve to distribute
physical stress over a larger area, protecting raised or recessed
brush features from abrasion and wear. The porosity of the skin may
also influence the movement of liquids through the brush, ensuring
the homogenous dispensing of cleaning fluids. The resilient skin
may be formed during or subsequent to a brush fabrication process
such as molding, extrusion, or milling, and can be accomplished
through the application of heat, chemicals, or radiation.
Inventors: |
McMullen, Daniel T.; (El
Dorado Hills, CA) ; Bahten, Kristan G.; (Gold River,
CA) ; Innes, James S.; (Cameron Park, CA) ;
Catalano, Vincent J.; (Folsom, CA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER
EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Family ID: |
25101114 |
Appl. No.: |
09/774398 |
Filed: |
January 30, 2001 |
Current U.S.
Class: |
15/102 ; 15/230;
15/230.16; 15/244.3; 15/244.4; 264/243 |
Current CPC
Class: |
H01L 21/67046 20130101;
B08B 1/04 20130101 |
Class at
Publication: |
15/102 ; 15/230;
15/230.16; 15/244.3; 15/244.4; 264/243 |
International
Class: |
B08B 001/04; B29D
031/00 |
Claims
What is claimed is:
1. A scrubbing device comprising: a shaped member comprising an
inner portion of a porous polymeric material and an outer surface
for removing residual particles from an object; and a resilient
skin overlying the outer surface, the resilient skin having a
higher density, a smaller average pore size, and a lower porosity
than the inner portion.
2. The scrubbing device of claim 1 wherein the porous polymeric
material comprises polyvinyl acetal material.
3. The scrubbing device of claim 2 wherein: the density of the skin
is greater than about 0.5 g/cm.sup.3, and the density of the inner
portion is less than about 0.1 g/cm; the average pore size of the
skin is about 60 .mu.m or less, and the average pore size of the
inner portion is about 100 .mu.m or greater; and the porosity of
the skin is approximately 50% or less, and the porosity of the
inner portion is about 80% or greater.
4. The scrubbing device of claim 1 further comprising a plurality
of nodules on the outer surface, the nodules covered with the skin
to prevent particulate contamination.
5. The scrubbing device of claim 1 wherein the member is cut from a
longer piece of porous polymeric material which results in an
exposed end of the inner porous polymeric material.
6. The scrubbing device of claim 5 wherein the end is covered by
the skin.
7. The scrubbing device of claim 1 wherein the skin is formed
following a molding process forming the member.
8. The scrubbing device of claim 1 wherein the skin is formed
following an extrusion process forming the member.
9. The scrubbing device of claim 1 wherein the skin is formed
following a milling process forming the member.
10. The scrubbing device of claim 1 wherein the porous polymeric
material comprises silicone material.
11. The scrubbing device of claim 1 wherein the porous polymeric
material comprises a polyurethane material.
12. The scrubbing device of claim 1 wherein the porous polymeric
material comprises a copolymer material.
13. The scrubbing device of claim 1 wherein said member is
cylindrical in shape.
14. A scrubbing brush comprising: an elongated member having an
exterior surface and an interior surface, the interior surface
defining a central bore extending at least part way through the
elongated member, the central bore shaped to receive a fluid
flowing member; a first skin disposed on the exterior surface and
characterized by a first porosity; a second skin disposed on the
interior surface and characterized by a second porosity, the first
porosity less than the second porosity to promote a flow of fluid
from the fluid flowing member across the second skin and to
minimize a pressure drop across the second skin.
15. The scrubbing brush of claim 14 wherein the second skin
features a surface roughness that enhances contact between the
brush and the fluid flowing member.
16. A scrubbing brush comprising an elongated member having an
exterior surface and an interior surface, the interior surface
defining a central bore extending at least part way through the
elongated member, the interior surface including a recess shaped to
receive a corresponding raised feature of a core structure.
17. The scrubbing brush of claim 16 wherein a resilient skin is
formed over the recess.
18. A scrubbing brush comprising an elongated member having an
exterior surface and an interior surface, the interior surface
defining a central bore extending at least part way through the
elongated member, the exterior surface including a reduced diameter
portion shaped to receive an arm for transporting a substrate.
19. The scrubbing brush of claim 18 wherein a resilient skin is
formed over the reduced diameter portion.
20. A method for making a scrubbing brush comprising; forming an
elongated master brush including a resilient outer skin; cutting
the elongated master brush to form a brush portion having an
exposed interior region lacking a skin; and sealing the interior
region with a second resilient skin, wherein the first skin and the
second skin are continuous with each other and form a substantially
particle free surface.
21. The method of claim 20 wherein sealing the interior region
creates an identification mark.
22. The method of claim 21 wherein creating an identification mark
comprises creating at least one of a part number, a part
manufacturer, and an applicable patent number.
23. A method for fabricating a compressive treatment brush material
comprising: transferring the molding substance to a mold; and
curing the molding substance within the mold to form a resilient
skin.
24. The method of claim 23 wherein the molding substance comprises
polyvinyl acetal (PVA).
25. The method of claim 24 wherein curing the molding substance
results in a skin having a density of greater than about 0.5
g/cm.sup.3, an average pore size of about 60 .mu.m or less; and a
porosity of about 50% or less, with an inner brush portion having a
density of about 0.1 g/cm.sup.3 or less, an average pore size of
about 100 .mu.m or greater, and a porosity of about 80% or greater.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to the manufacture of objects.
More particularly, the present invention provides a device for
cleaning substrates used in electronics, such as in the fabrication
of integrated circuits from substrates of semiconductor materials.
By way of example, the present invention is applied to the
manufacture of substrates for integrated circuits, but it will be
recognized that the invention has a wider range of applicability
and can also be applied to the manufacture of other types of
substrates such as memory hard disks, flat panel displays, and the
like.
[0002] In the manufacture of electronic devices, the presence of
particulate contamination is a serious issue. Particulate
contamination can cause a wide variety of problems such as
mechanical and/or electrical failures. These failures often take
the form of reliability and functional problems of electronic
devices formed on a substrate. For example, particulate
contamination is one of the main sources for lower device yields,
increasing the cost of an average functional IC being
manufactured.
[0003] Particulate contamination can be introduced to the substrate
during the fabrication process, for example during the application
of abrasive slurry during chemical mechanical planarization (CMP)
steps, or during etching processes which leave behind unwanted
residue. Each of these processes often introduce the aforementioned
impurities onto the surface of the substrate. These impurities
generally become bound to the substrate and must often be
removed.
[0004] A variety of techniques have been used or proposed to remove
the impurities. One technique is a conventional rinser, which uses
a cascading rinsing fluid such as deionized water to carry away
particulate contamination. The cascade rinse utilizes a rinse tank
which includes inner and outer chambers, each separated by a
partition. In most cases, rinse water flows from a water source
into the inner chamber. The rinse water from the inner chamber
cascades into the outer chamber. An in-process substrate is
typically rinsed in the cascade rinser by dipping it into the rinse
water of the inner chamber. A limitation with the cascade rinser is
that "dirty water" often exists in the first chamber. The dirty
water typically has "particles" which can attach themselves to the
substrate. These particles often cause defects in the substrate,
thereby reducing the number of defect-less substrates in the
manufacturing process. Another limitation with the cascade rinser
is that particles having a strong attraction to the substrate
cannot be removed by the rinse fluid. Accordingly, the cascade
rinse often cannot remove particles from the substrate.
[0005] An alternative technique for removing particles is a
scrubbing process. The scrubbing technique uses a scrubber with
scrubbing brushes or rollers. An example of a scrubber that uses
scrubbing brushes is the Synergy.TM.CMP cleaning system
manufactured by Lam Research Corporation of Fremont, Calif. This
scrubber has the pair of scrubbing brushes that are cylindrical in
shape. The brushes are biased against a substrate and rotated to
remove particles.
[0006] Although the aforementioned scrubbing brushes have been
partly effective in removing particles and/or contamination, a
variety of limitations still exist with their use in conjunction
with the manufacture of conventional semiconductor substrates. One
such limitation is introduction of particle contamination to the
substrate from exposed ends of a scrubbing brush.
[0007] FIG. 1 shows a simplified perspective view of a brush
scrubbing apparatus in which arm 10 engages edge 12a of substrate
12 and moves the substrate laterally along length 14 between
rotating brushes 16. Brushes 16, however, are typically formed by
cutting a longer piece of polyvinyl acetal (PVA) material, such
that brush interior 16a having a porous cell-type structure is
exposed at the end. Because of its open cell type structure, brush
interior 16a may be prone to shed particles onto substrate 12 as
the substrate passes by the end of the brush.
[0008] Moreover, brushes 16 typically include a plurality of
projecting nodules 18. These nodules are generally already formed
as a part of the master PVA member from which brush 16 is cut. If
the master PVA piece is cut into sections at nodules 18, only a
part of a nodule may remain on the exterior surface of the brush.
This cutting may thus weaken the nodule structure, such that the
nodule may be fragile and contribute particulate contamination to
the passing substrate.
[0009] Based upon the above, it is seen that an improved material
for treatment of substrates is highly desired.
SUMMARY OF THE INVENTION
[0010] Embodiments of the present invention provide a device for
treatment of surfaces of substrates, for example as are utilized in
the formation of integrated circuits and data storage. In one
exemplary embodiment, the present invention provides an improved
scrubbing device which includes a resilient skin that encloses a
porous polymeric interior portion. The resilient skin is of
controllable thickness, structure, and texture, and overlies the
exterior surface of the brush, including any projecting nodules.
The skin thus isolates interior porous brush portions from contact
with the substrate and maintains intact nodule portions resulting
from fabrication of the brush.
[0011] One embodiment of a scrubbing device in accordance with the
present invention comprises a shaped member comprising an inner
portion of a porous polymeric material, and an outer surface for
removing residual particles from an object. A resilient skin
overlies the outer surface, the resilient skin having a higher
density and a lower porosity than the inner portion.
[0012] One embodiment of a method for making a scrubbing brush
comprises forming an elongated master brush including a resilient
outer skin, and cutting the elongated master brush to form a brush
portion having an exposed interior region lacking a skin. The
interior region is sealed with a second resilient skin, wherein the
first skin and the second skin are continuous with each other and
form a substantially particle-free surface.
[0013] These and other embodiments of the present invention, as
well as its advantages and features are described in more detail in
conjunction with the text below and attached Figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 shows a simplified perspective view of a conventional
scrubbing apparatus in which a substrate is passed along a length
of a scrubbing brush.
[0015] FIG. 2 shows a simplified exploded view of a scrubbing brush
bearing a resilient skin in accordance with one embodiment of the
present invention.
[0016] FIG. 3 shows a simplified cross-sectional view of a
scrubbing brush bearing a resilient skin in accordance with an
alternative embodiment of the present invention.
[0017] FIG. 4 shows a simplified scanning electron microscope image
of the cross-section of the interface between the resilient skin
and an internal porous portion of a brush in accordance with one
embodiment of the present invention.
[0018] FIG. 5A shows a side view of an alternative embodiment of a
brush in accordance with the present invention which features a
reduced diameter portion.
[0019] FIG. 5B, which is a cross-sectional view of FIG. 5A along
line 5B-5B'.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
[0020] FIG. 2 shows a simplified exploded view of a scrubbing brush
in accordance with one embodiment of the present invention. Brush
200 comprises a porous polymeric material 202 encased within first
resilient skin portion 204. Porous polymeric material 202 may be
formed from polyvinyl acetal, polyurethane, silicone and other
polymer or copolymer materials.
[0021] Bore 203 extends through porous polymeric material 202.
Annular ends 201 of brush 200 are encased within a second, annular
resilient skin portion 205. Outer surface 200a of brush 200
includes a plurality of projecting nodules 206 also covered by
resilient skin portions 204 and 205 as shown.
[0022] Porous polymeric material 202 can be composed of polyvinyl
acetal, polyvinyl chloride, polyurethane, silicone, or any other
suitable porous polymeric material compatible with the substrate to
be treated and the chemistry being utilized to treat the substrate.
As described above, porous polymeric material 202 typically
exhibits a cell-type structure.
[0023] The resilient skin exhibits a different physical structure
than the interior porous portion of the brush. Specifically, the
difference in physical structure can be characterized in several
ways, including increased density, lower porosity, and smaller pore
size.
[0024] One characteristic of the resilient skin material is
density. In accordance with one embodiment of the present
invention, a PVA brush includes a resilient skin having a density
of between about 0.5 to 1.3 g/cm.sup.3. By contrast, the density of
the interior of the PVA brush is between about 0.07 to 0.10
g/cm.sup.3.
[0025] Another characteristic of the skin material is pore size.
Pore size reflects the size of the pores in the material, and may
be measured from high power photographs of cross-sections of the
material. FIG. 4 shows a scanning electron micrograph (SEM) image
at a power of 30 kV of a cross-section of a PVA brush at the
interface between skin 400 and the porous brush interior 402. Such
a skin may be formed with a cure temperature range of between
40-220.degree. C. for a period of from on the order of minutes to
days.
[0026] FIG. 4 shows that the structure of brush interior 402
includes large pores 402a separated by struts 404. Pores 402a of
brush interior 402 are much larger than pores 400a of skin 400.
Both of these factors reveal the significantly smaller pore size of
skin 400 versus brush interior 402. For example, where the
scrubbing brush is formed from polyvinyl acetal, the average pore
size of the porous polymeric interior is between about 110 and 150
.mu.m, while the average pore size of the brush skin is between
about 40 and 60 Another characteristic of the resilient skin
material is porosity. Porosity describes the percentage of the
volume of the brush occupied by the pores, and may be measured by
perfusion of helium through the material. Porosity of the resilient
skin of a brush formed from polyvinyl acetal is about 50%. By
contrast, porosity of the interior PVA material of the brush was
between about 80-95%.
[0027] The above-referenced characteristics of the resilient skin
can offer significant advantages during cleaning processes.
[0028] For example, one important role of the brush is to exert
physical force against the surface of the substrate, either
directly through contact with the substrate, or indirectly through
a lubricating (cleaning) fluid. However, the cell struts of the
brush interior shown in FIG. 4 are not especially durable under
concentrated load or abrasion conditions. The increased density of
the resilient skin material thus may play an important role in
distributing surface stresses over a larger area of the brush.
Absent the resilient skin, individual struts forming the walls of
the cells of the porous polymeric interior would likely be exposed
to more intense forces than where the dense skin distributes the
force. Struts of a skinless brush would thus be more susceptible to
wear resulting from abrasion. Such wear could lead to contamination
of the substrate by fragments of the brush interior.
[0029] Another role of the brush during cleaning may be to dispense
liquid cleaning material. This is described in detail in co-pending
U.S. Nonprovisional patent application No. 09/586,665, filed Jun.
1, 2000 and hereby incorporated by reference. Accordingly, another
advantage of resilient skin is that its porosity can be used to
influence the flow of fluids through the brush. This aspect of the
present invention is illustrated below in conjunction with FIG.
3.
[0030] FIG. 3 shows a simplified cross-sectional view of a
scrubbing brush bearing a resilient skin in accordance with an
alternative embodiment of the present invention. Brush 300 is
cylindrical in shape and includes an exterior surface 302 bearing
nodules 304, and an interior surface 306 defining bore 308.
Internal porous polymeric material 305 having a typical open-celled
structure is present between exterior surface 302 and interior
surface 306.
[0031] Exterior surface 302 and nodules 304 of brush 300 are
covered with first resilient skin portion 310 having a first
porosity. Interior surface 306 is covered with second resilient
skin portion 312 having a second porosity. During cleaning, a fluid
may flow out of openings 314a in fluid flow member 314, and through
second skin 312 into internal porous polymeric region 305. Once
internal region 305 becomes saturated with fluid, the fluid flows
out of first skin portion 310 to come into contact with a
substrate.
[0032] By having second skin portion 312 exhibit greater porosity
than first skin portion 310, the pressure drop across second skin
312 is lessened and fluid is able to enter brush interior 305 more
easily. Moreover, less porous first skin 310 serves as a membrane
to evenly distribute the flow of liquid through brush interior 305,
resulting in even distribution of fluid delivered to the substrate
through the brush.
[0033] Physical characteristics of the skin material may also
facilitate implementation of texture on the brush. As described
below, where the skin is formed by molding, texture on the surface
of the mold may be transferred to the surface of the molded part.
Skin texture can be useful in a number of applications. For
example, during installation of a brush on a scrubbing apparatus,
the bore is typically slipped around the end of a projection and
secured by frictional contact. Enhanced roughness of an interior
skin portion of the brush may aid in securing the brush to the
scrubbing device.
[0034] A textured brush skin could also be used to influence
friction factors and fluid dynamics on a microscopic level at the
brush-substrate interface. Properties such as shear forces, mass
transport, and surface chemistry could be influenced and controlled
by the texture of the brush skin.
[0035] The coarseness of a textured brush skin could be tailored to
achieve optimum results in removing particle contamination from a
given type of substrate. The texture of the resilient skin can vary
in roughness, and can be random or patterned in nature. In one
embodiment of the present invention, the location and character of
the texturing could be determined by the presence of corresponding
textured features on the surface of the mold. Optimal skin texture
for a given brush could be a function of 1) the material properties
of the brush, 2) the nature of the substrate being treated, 3) the
chemistry being utilized to treat the substrate, 4) the nature of
the materials sought to be removed from the substrate, and 5)
operational parameters for the specific scrubbing device.
[0036] Alternatively, or in conjunction with a brush having
enhanced surface roughness, a brush could be shaped by molding,
milling, or extrusion to include an interior having recesses
configured to receive corresponding raised features on the outside
surface of a core or mandrel portion. In this manner, the interior
shape of the brush could be keyed to the exterior shape of the core
or mandrel in order to ensure proper alignment of the brush, and to
prevent bunching or slippage of the brush on the core during use.
The presence of a resilient skin overlying the recesses in the
brush would help strengthen the brush and prevent particulate
contamination from recess-edge brush regions subject to additional
stress because of their shape.
[0037] Alternatively, or in conjunction with a brush featuring a
shaped inner surface, a brush in accordance with the present
invention could also exhibit a shaped outer surface. FIG. 5A shows
a side view of an alternative embodiment of brush 500 in accordance
with the present invention, which features a reduced diameter
portion 502. Reduced diameter portion 502 allows for the insertion
of arm 504 carrying disk-like substrate 506 at contact point
508.
[0038] The role of reduced diameter portion 502 is further
illustrated in connection with FIG. 5B, which is a cross-sectional
view of FIG. 5A along line 5B-5B'. Arm 504 moves downward carrying
substrate 506, and places substrate 506 between nodules 510 of
brush 500 and second brush 512. Upon release of substrate 506 from
arm 504 and withdrawal of arm 504 from reduced diameter portion
502, rotation of brushes 500 and 512 cleans the substrate and
causes the substrate to move horizontally along the length of the
brushes. The presence of the resilient skin over the reduced
diameter portions of the brush will strengthen these portions
against degradation and particle contamination caused by
stress.
[0039] The resilient skin material of the present invention may be
formed by a number of processes. Where the brush is formed by
molding, the resilient skin may result from thermal interaction
between the surface of the mold and precursor polymeric material
within the mold. In such a case, a temperature differential at the
point of contact between the mold and the polymeric material could
result in an increased density and decreased porosity of the molded
material itself. Alternatively, the resilient skin may be produced
by coating an interior mold surface with a separate material that
is itself thermally cured or reacts with the porous polymeric
material to form the resilient skin during the molding process. The
materials of construction chosen for the mold can have a
significant impact on the resulting skin. Surface interactions
between the mold interface and the polymer mix will affect the
wetting of the mold and/or act as a site for skin formation.
[0040] A scrubbing brush having a resilient skin may also be formed
using an extrusion process. Where extrusion is used to form the
brush, the porous polymeric material may be extruded through an
opening covered by a film comprising the skin material, in a manner
analogous to formation of skin around a sausage.
[0041] A scrubbing brush having a resilient skin may also be
machined from a block of foam. In contrast with molded parts
typically having softened, rounded edges, brushes formed by
machining typically have relatively sharp edges that are
susceptible to wear. As a result of this wear, particle
contamination may be enhanced for substrates that are in contact
with these edges. Thus in accordance with embodiments of the
present invention, the resilient skin, or portions of the resilient
skin, may be added subsequent to brush formation processes such as
milling.
[0042] In one embodiment of the present invention, the skin may be
produced by applying a protective coating to exposed exterior
surfaces of the already-formed brush. In an alternative embodiment
of the present invention, a first portion of the skin may be
created during formation of a master elongated porous polymeric
piece, with the second skin portion formed over ends of the brush
section exposed by cutting of the master elongated polymeric piece.
In such an alternative embodiment the skin may cover the exposed
ends of the porous polymeric member (as shown by second, annular
resilient skin portion 205 of FIG. 2) thereby preventing
particulate contamination from occurring from these ends.
[0043] Where the skin is formed following a molding process, the
skin may result from chemical interaction with the existing molded
porous polymeric material, for example by dipping the molded member
into a chemical bath. Alternatively, the skin may form through
thermal interaction, for example by heating the molded member in a
furnace, by cutting the shorter piece from the elongated master
using a heated wire, or by applying a heated cap to the exposed
ends to produce a cauterizing effect. Such an end cap could further
be engraved with an identification mark such as a part number, a
part manufacturer, and/or an applicable patent number, offering yet
another advantage of the present invention.
[0044] In addition to exposure to heat or chemicals, the skin may
be formed through other processes, such as interaction with
electromagnetic radiation, for example from application of a laser
beam.
[0045] The above description is illustrative and not restrictive,
and as such should not be limiting to the claims as described
herein. While the above embodiments are generally described in
terms of use in manufacturing semiconductor substrates, the
invention has a much broader range of applicability. For example,
the present invention could be applied to brushes used in a
manufacturing process for flat panel displays, optical devices, and
other devices requiring a high degree of cleanliness.
[0046] Moreover, while the resilient skin is described in the above
embodiments primarily in terms of porosity and roughness, other
attributes could be utilized to describe the resilient skin. For
example, during a brush compression-relaxation event, the
"resiliency bounce" of a skin material describes the ability of the
material to dissipate compression energy versus the amount of
energy transferred back to the surroundings upon relaxation of the
brush after compression. One method of measuring resiliency bounce
is set forth by American Society for Testing and Materials (ASTM)
test D 3574, wherein a steel ball of known diameter and weight is
dropped from a fixed height onto a specimen, and the height of the
rebound measured as a percentage of the initial drop height. For
applications involving cleaning of electronic substrates, a
resiliency bounce of between 25-30% is preferable.
[0047] Another physical property characterizing the skin material
is determination of the temperature and frequency dependence of the
storage modulus, the loss modulus and the mechanical loss factor.
These properties may be measured through Dynamic Mechanical
Analysis (DMTA) utilizing the Advanced Rheometric Expansion System
(ARES), manufactured by Rheometrics Scientific of Piscataway, N.J.
If the time for full recovery of the skin material from a
compression event is not sufficiently short, the material would be
unsuitable for use in a particular application requiring repeated
compression and expansion of the brush.
[0048] Yet another physical property characterizing the skin
material is recovery time or hysteresis. Material recovery time or
hysteresis should ideally be less than the time that it takes for
the brush to undergo one full rotation. A brush rotating at 50 rpm
would thus optimally have a recovery time of less than 0.02
minutes. A brush rotating at 1200 rpm would thus optimally have a
recovery time of less than 8.3.times.10.sup.-4 minutes. This
recovery time is a function of the elastic properties of the skin
material as well as the centrifugal force operating to expand the
brush outward as it rotates. The fluid present inside the brush
while it is spinning increases this centrifugal force. The porosity
or permeability of the skin will also influence the rate at which
the material recovers from the centrifugal force due to the amount
of resistance applied as the water flows through the skin. A more
porous skin material may offer less resistance to the flow of
fluid, reducing the impact of the centrifugal force on the membrane
and counterbalancing material recovery time.
[0049] Compression set is another important property of the skin
material. Compression set characterizes the degree to which the
material recovers its original shape following a compression cycle.
In order for a porous polymeric brush to be effective in a cleaning
process, it must remain in contact with the substrate. Therefore, a
compressive set of the material should be less than the normal
compression the brush experiences under ordinary usage conditions.
The compressive set should not exceed about 50% of the normal
compression the brush experiences during the cleaning process. For
example if a brush is compressed 2 mm from the relaxed state when
in contact with the substrate, the compressive set of the brush
skin material should be less than 1 mm. A steady state value for
the compressive set should be achieved early in the cleaning
process, or within about the first 25% of the cycle time for
cleaning each individual substrate.
[0050] At high rotation speeds, the brush may swell due to the
centrifugal forces exerted on the brush and liquid material
contained within the brush. Elasticity and tensile strength of the
skin play an important role in determining the shape and degree of
deformation of the brush when it is rotating. Below a minimum
tensile strength of the skin, the skin material may be torn apart.
Above a maximum tensile strength of the skin, the skin material may
be too rigid and fail to conform to the surface of the substrate
being cleaned.
[0051] While the above is a full description of the specific
embodiments in accordance with the present invention, various
modifications, alternative constructions and equivalents may be
used. Therefore, the above description and illustrations should not
be taken as limiting the scope of the present invention which is
defined by the appended claims.
* * * * *